Conventionally, an EGR system (exhaust gas recirculation apparatus) is provided to an internal combustion engine such as a diesel engine. The EGR system is adapted to recirculating a part of exhaust gas (EGR gas) of the engine from an exhaust passage to an intake passage so as to reduce emission of the engine. EGR gas contains a large amount of inert gas such as steam, carbon dioxide, and the like after combustion. The EGR system is capable of effectively reducing combustion temperature of the engine, thereby effectively reducing a toxic substance such as nitrogen oxide (NOx) contained in exhaust gas.
The EGR system includes an EGR flow control valve (EGRV) midway through an EGR pipe, which leads EGR gas from the exhaust passage to recirculate the EGR gas to the intake passage. The EGRV controls an amount of the EGR gas returning to the intake passage through an GFR passage in the EGR pipe. The EGR system includes an exhaust gas cooler (EGR cooler) midway through the EGR pipe. The EGR cooler may be a water cooling type heat exchanger. The EGR cooler cools EGR gas, which returns to the intake passage, thereby enhancing a charging efficiency of the engine. Thus, emission of the engine can be effectively reduced.
In the above cooled EGR system, EGR gas is cooled through the EGR cooler, and the cooled gas is recirculated into the intake passage. In view of further emission regulations of an engine such as a diesel engine, a hot EGR system is further needed in addition to the cooled EGR system. The hot EGR system is adapted to leading EGR gas as hot EGR gas to bypass the EGR cooler, and returning the hot EGR gas into the intake passage. The hot EGR system is capable of enhancing combustion in the engine when the engine is started in the cold condition or in regeneration of a diesel particulate filter (DPF).
Specifically, the hot EGR system has a bypass passage in parallel with an EGR passage. EGR passage leads EGR gas from the exhaust passage to recirculate the EGR gas to the intake passage through the EGR cooler. The bypass passage leads EGR gas to bypass the EGR cooler. The hot EGR system is adapted to recirculating EGR gas to the intake passage without passing through the EGR cooler in an engine starting or the like.
Regeneration of the DPF is performed by supplying hot exhaust gas to the DPF so as to heat the DPF such that temperature of the DPF becomes greater than combustion temperature of a particulate matter (PM). The hot EGR system is capable of leading hot EGR gas into the intake passage, thereby enhancing heating of the DPF in regeneration. The hot EGR system is also capable of increasing temperature of intake air drawn into the combustion chamber of the engine. Therefore, the hot EGR system is capable of further effectively heating exhaust gas supplied to the DPF, thereby further effectively reducing emission of the engine.
According to EP0987427, an EGR system is capable of manipulating a flow of cooled EGR gas and a flow of hot EGR gas so as to control temperature of EGR gas returning to an intake passage.
As shown in FIG. 9, the EGR system includes an EGR module constructed of an EGR cooler 101, a housing 102, two passage selector valves (valve plates) 103, 104, a rotation axis 105, and a negative pressure controlled actuator 106. The EGR cooler 101 cools EGR gas using engine cooling water. The housing 102 has therein two first and second valve chambers. The two passage selector valves (valve plates) 103, 104 are respectively accommodated in the first and second valve chambers of the housing 102. The rotation axis 105 supports the passage selector valves 103, 104. The negative pressure controlled actuator 106 drives the rotation axis 105 to manipulate the passage selector valves 103, 104. The EGR cooler 101 has a U-shaped EGR passage having two parallel passages connected via a U-shaped portion. The passage selector valve 103 is coupled to the rotation axis 105 such that the passage selector valve 103 accommodated in the first valve chamber and the passage selector valve 104 accommodated in the second valve chamber form a relative angle (phase difference) of 70 to 90°.
In the present structure, a branch pipe is provided upstream of the housing 102, and a junction pipe is provided downstream of the housing 102 with respect to the flow direction of EGR gas. The housing 102 of the two passage selector valves 103, 104 does not have a branch portion and a junction portion, i.e., merge portion.
The housing 102 has a cooler mount face, a branch-pipe mount face, and a junction pipe mount face respectively provided with two of six EGR ports 111 to 116. The EGR ports 111, 112 are communicated with the first valve chamber. The EGR ports 115, 116 are communicated with the second valve chamber.
The housing 102 has an EGR passage and a bypass passage 123 adjacently extending in parallel with each other. The EGR passage (main passage) leads exhaust gas of the internal combustion engine so as to recirculate the exhaust gas to the intake passage through the EGR cooler 101. The bypass passage 123 leads exhaust gas from the internal combustion engine to recirculate the exhaust gas to the intake passage through the second valve chamber so as to bypasses the EGR cooler 101.
The EGR passage has a cooler inlet gas passage 121 and a cooler outlet gas passage 122. The cooler inlet gas passage 121 leads hot EGR gas discharged from the internal combustion engine into the EGR cooler 101 through the first valve chamber. The cooler outlet gas passage 122 recirculates cooled EGR gas, which is cooled through the EGR cooler 101, to the intake passage.
A partition 124 connects the cooler mount face, to which the EGR cooler 101 is attached, with the passage wall surface defining the bypass passage 123. The partition 124 divides the interior of the housing 102 into the EGR passage and the bypass passage 123. The EGR passage includes the cooler inlet gas passage 121 and the cooler outlet gas passage 122.
In view of mountability to an engine, an EGR module is needed to be downsized by integrating an EGR cooler, a passage selector valve, and EGRV. The EGR module disclosed in EP0987427 has the two passage selector valves 103, 104 provided in the two parallel passages, and the two passage selector valves 103, 104 are connected with the single rotation axis 105. In the present structure, the EGR module is enlarged, and hence mountability to a vehicle such as a car, in particular, an engine is impaired.
In addition, in the EGR module of EP0987427, the cooler mount face is directly connected with the passage wall surface of the bypass passage 123 via the partition 124 of the housing 102. In addition, the rotation axis 105 of the two passage selector valves 103, 104 is in parallel with the axes of the EGR cooler 101 and the housing 102 passing through the center of the cooler mount face. One end of the rotation axis 105 is rotatably supported by a bearing 125 provided in the vicinity of the cooler mount face.
In the present structure, the bypass passage wall surface is directly exposed to hot EGR gas passing through the bypass passage 123, and accordingly, temperature of the bypass passage wall surface increases. The rotation axis 105 and the partition 124 of the housing 102 may be formed of a thermally conductive material. In EP0987427, the housing 102 is formed of an aluminum material. In this case, temperature of the partition 124 of the housing 102 significantly increases due to thermal conduction from hot EGR gas.
When temperature of the partition 124 of the housing 102 increases, temperature of the cooler mount face of the housing 102 also increases. Consequently, temperature of the EGR cooler 101 increases due to heat conduction via the cooler mount face of the housing 102. That is, in the structure of EP0987427, heat of hot EGR gas passing through the bypass passage 123 is easily conducted to the EGR cooler 101 of the EGR module.
When the hot EGR mode is switched to the cooled EGR mode by manipulating the rotation angle of the two passage selector valves 103, 104, cooled EGR gas is recirculated to the intake passage through the EGR cooler 101. In this condition, even engine cooling water is recirculated inside the EGR cooler 101, cooling performance of the EGR cooler 101 is impaired due to thermal conduction from hot EGR gas in the hot EGR mode. As a result, emission cannot be sufficiently reduced.
In addition, when cooled EGR gas and hot EGR gas are mixed and returned to the intake passage so as to control temperature of EGR gas corresponding to the operating condition of the engine, the partition 124 of the housing 102 is transmitted with heat from hot EGR gas passing through the bypass passage 123. Accordingly, the hot EGR gas exerts influence to cooled EGR gas passing through the cooler outlet gas passage 122. Consequently, temperature of the cooled EGR gas passing through the cooler outlet gas passage 122 increases. Accordingly, it is hard to control temperature of mixture of hot EGR gas and cooled EGR gas, which returns to the intake passage. As a result, emission cannot be sufficiently reduced,